From Bulbapedia, the community-driven Pokémon encyclopedia.
The Great Marsh (Japanese: だいしつげん) is an area located in Pastoria City. It contains some Pokémon that can not be found anywhere else in Sinnoh. It seems to be the Sinnoh region's counterpart to Kanto and Hoenn's Safari Zones and Johto's National Park. It is also the only Safari Zone to feature a train ride, which can only go one straight direction, whilst journeying through the six areas of the Great Marsh.
The Great Marsh has also a new cool spin in looking for wild Pokémon. The counted-steps mechanic is now featured with a brand new twist; players could get stuck on muddy grass (a first in Diamond and Pearl) if he/she moves too slowly or passes through mud-tiles with no grass at all, therefore leading the player to get himself/herself out. Every attempt of the player to get himself/herself out counts as a deduction to the given step-count.
These Pokémon will not always appear in the Marsh. Each day, the Pokémon will change, therefore, to get a new set, players must wait a day. These Pokémon appear randomly, so some may not appear for a while, while others appear several days in a row. Another note is that some may not appear until after getting the National Pokédex.
This list is not yet complete.
Before National Pokédex
After National Pokédex
GBA Cartridge Insertion